1. Field of the Invention
The present invention relates to a circuit pattern detecting apparatus and a circuit pattern inspecting method, which optically check the circuit pattern of a circuit board.
2. Description of the Related Art
As a conventional technique for checking whether the circuit pattern on a circuit board contains a disconnected or short-circuited portion, a technique that uses a dedicated jig is known. The jig is formed by use of spring probes and the pads of the circuit pattern are simultaneously brought into electric contact with the jig. This technique is not suitable for inspecting recently-developed circuit patterns having an increased number of pads since a large number of spring probes, which are expensive, are needed and the manufacturing cost of jig is inevitably high. In addition, since the pads are arranged with high density, reliable physical contact with them is hard to attain. The pads may be damaged if sharp spring probes are brought into contact with them.
Furthermore, in some circuit boards, one pad is a junction from which different circuit patterns branch off and is therefore connected to a plurality of other pads. In this case, the time needed for inspecting disconnected or short-circuited circuit portions may be very long.
Furthermore, although many circuit patterns may be formed in layers in some circuit boards, the technique for bringing the spring probes into contact with the pads cannot detect disconnected or short-circuited portions in such circuit patterns.
Under the circumstances, a technique for optically detecting the voltage distribution in a circuit pattern has been desired. As a conventional technique which measures the voltage distribution in a circuit pattern by utilization of the electro-optic effect, Japanese Patent Publication (KOKAI) No. 9-72947 discloses a known solder connection detection method and detection apparatus for electronic parts. According to the publication, the electric field strength of a predetermined position is detected in a non-contact manner by means of an electro-optic sensor, and the solder connection state of the circuit board is examined. However, the method can detect only the electric field at the tip end of the electro-optic sensor, and the electro-optic sensor has to be scanned so as to obtain the voltage distribution of the entire circuit pattern.
On the other hand, Japanese Patent Publication (KOKAI) No. 5-256794 describes an apparatus which detects disconnected, short-circuited or other defective portions in a pixel electrode, gate wiring and source wiring of a liquid-crystal display panel, by measuring the voltage distribution in a non-contact manner. According to this publication, an electro-optic device located near the circuit board is irradiated with a collimated beam, and the voltage distribution in a circuit pattern is detected in two dimensions based on the reflected light.
However, since the index of birefringence of an electro-optic device is high, interference fringes are inevitably produced due to the interference between the light reflected by the obverse surface of the device and the light reflected by the reverse surface. As a result, a voltage distribution image based on the reflected light deteriorates significantly.
Moreover, when a voltage is applied to the circuit pattern of a circuit board, electric charges tend to diffuse in the plane direction of the electro-optic device, deteriorating the voltage distribution. In other words, when voltage is applied to the circuit pattern of the circuit board, the voltage distribution of the circuit pattern may undesirably spread to the electro-optic device, due to the DC resistance components in the plane direction, especially the DC resistance components which the reflecting layer of the electro-optic device may have.
An object of the present invention is to provide a circuit pattern detecting apparatus and a circuit pattern detecting method, which optically detect the voltage distribution of a circuit pattern on a circuit board with high accuracy and which inspect disconnected or short-circuited portions of the circuit pattern.